rock and earth formations to be met. However, had more
attention been paid to geologic questions in the planning stages, this
great undertaking, so thoroughly worked out from a purely engineering
standpoint, would have avoided certain mistakes due to lack of
understanding of the geological conditions. It is a curious fact that in
these early stages no strength tests of rocks were made, and that no
thorough detailed study was made of the geologic factors affecting
slides and their prevention. It was only after the slides had become
serious that the geological aspects of the subject were intensively
considered. The results of the geologic study, therefore, are useful
only for preventive measures for the future and for other undertakings.
One of the interesting features of this investigation was the discovery
that certain soft rock formations were rendered weaker rather than
stronger by the draining off of the water. It had been more or less
assumed that the water had acted as a lubricant rather than as a cement.

SUBSIDENCE

Not the least important application of geology to slides is in relation
to deep mining operations. While the mining geologist has been
principally engaged in exploration and development of ores, he is now
beginning to be called in to interpret the great earth movements caused
by the sinking of the ground over mining openings. For instance, the
long-wall method of coal mining has resulted in a slow progressive
subsidence of the overlying rock, affecting overlying mineral beds and
surface structures over great areas. Detailed studies have been made of
this movement, in order to ascertain its relation to the strength and
structure of the rocks, its relation to the nature of the excavation,
its speed of transmission, and the possible methods of prevention.
German scientists have perhaps gone further with this kind of study than
anyone else. In an elaborate investigation of subsidence over a coal
mine in Illinois,[67] unusually complete data were obtained as to the
nature, direction, and speed of the transmission of strains through
large rock masses, and as to their effect in producing secondary rock
structures.

RAILWAY BUILDING

In railway building, the planning and estimation of cuts and fills is
now receiving geologic consideration, in order to make sure that no
geologic condition has been overlooked which will affect costs, the
stability of the road, or the accurate formulation of contracts.